This invention pertains to self-dispersing pigment dispersions which are modified with a multivalent cation and an anionic polymer.
Inkjet printing is a non-impact printing process in which droplets of ink are deposited on a substrate, such as paper, to form the desired image. Inkjet printers are equipped with an ink set which, for full color printing, typically comprises a cyan, magenta and yellow ink (CMY). An ink set also commonly comprises a black ink (CMYK).
Pigments suitable for aqueous inkjet inks are in general well-known in the art. Typically, pigments are stabilized by dispersing agents, such as polymeric dispersants or surfactants, to produce a stable dispersion of the pigment in the vehicle. The pigment can also be “self-dispersible” or a “self-dispersing” pigments (hereafter “SDP(s)”).
U.S. Pat. No. 6,899,754 discloses an inkjet ink containing an SDP and a multivalent cation. The multivalent cation is added at the ink formulation stage rather than during the preparation of the SDP dispersion.
U.S. Pat. No. 6,332,919 and EP-A-1086997 disclose a black inkjet ink comprising an SDP and salts of monovalent cations. It is suggested that the presence of these monovalent salts improves optical density at a given pigment loading.
U.S. Pat. No. 6,277,183 discloses a black inkjet ink comprising an SDP ink and a metal oxide, where optical density of the ink is higher when metal oxide is present than when it is absent.
U.S. Pat. No. 6,153,001 discloses an example of a black inkjet ink containing an SDP (Microjet® CW1) and 9 ppm calcium. No information is provided on the source or physical state of the calcium or on the nature of the SDP. No suggestion is made of any optical density relationship.
U.S. Pat. No. 6,375,317 discloses an inkjet ink comprising an SDP and calcium hydroxide in an aqueous medium.
A need still exists for stable self-dispersing pigment dispersions that can be readily formulated into an ink. The present disclosure satisfies this need by providing a process for preparing stable self-dispersing pigment dispersions.